U.S. patent number 5,637,809 [Application Number 08/456,650] was granted by the patent office on 1997-06-10 for vacuum extraction sampling system.
This patent grant is currently assigned to United Sciences, Inc.. Invention is credited to Richard Myers, John E. Traina.
United States Patent |
5,637,809 |
Traina , et al. |
June 10, 1997 |
Vacuum extraction sampling system
Abstract
A gas sampling system utilizes small sample vacuum transport to
reduce the dew point of the sample. A vacuum pump maintains a
substantial vacuum on the sampling system causing a sample, drawn
at a rate less than a liter per minute, to be drawn and transported
under partial vacuum for analysis. A dryer can be placed near the
sampling probe to further reduce the dew point prior to the vacuum
transport. The dew point of the sample is affected by both the
dryer and the degree of vacuum transporting the gas mixture. As
such, the dew point can be varied indefinitely by any reasonable
combination of moisture removal by the dryer and vacuum pump
strength.
Inventors: |
Traina; John E. (Glenshaw,
PA), Myers; Richard (Gibsonia, PA) |
Assignee: |
United Sciences, Inc.
(Gibsonia, PA)
|
Family
ID: |
26913039 |
Appl.
No.: |
08/456,650 |
Filed: |
June 2, 1995 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
218563 |
Mar 28, 1994 |
|
|
|
|
789935 |
Nov 12, 1991 |
5297432 |
Mar 29, 1994 |
|
|
Current U.S.
Class: |
73/864.12;
73/864.34 |
Current CPC
Class: |
G01N
1/2258 (20130101); G01N 33/0011 (20130101); G01N
33/0016 (20130101); G01N 1/2252 (20130101); G01N
1/24 (20130101); G01N 2001/2261 (20130101); G01N
2001/2264 (20130101); G01N 2001/244 (20130101) |
Current International
Class: |
G01N
33/00 (20060101); G01N 1/22 (20060101); G01N
1/24 (20060101); B01L 007/00 () |
Field of
Search: |
;73/864,864.12,864.22,864.34,864.35,29.01 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Richard L. Myers and Donald Vernon, "Field Experiences Using
Dilution Gas Probe Techniques for Continuous Source Emission
Monitoring," Proceedings of the Controls West Conference, pp.
347-355, submitted to the International Industrial Controls
Conference and Exhibition/Controls West '85, Long Beach Convention
Center, Long Beach, California, Sep. 16-18, 1985. .
"Model 797: Diluting Stack Sampler," EPM Environmental Product
Brochure. .
Patent Abstrafts of Japan, vol. 009, No. 320, (P-413) 14 Dec., 1985
& JP-A-60 147 635 (Toyota Jidosha KK) 3 Aug. 1985. .
Patent Abstracts of Japan, vol. 011, No. 354, (P-638) 19 Nov., 1987
& JP-A-62 133 336 (Nippon Soken, Inc., Others: 01) 16 Jun.,
1987. .
"Nafion Gas Sample Dryers" brochure, undated, received early 1995.
.
"Chapter 6: Extractive System Design," EPA Handbook: Continuous Air
Pollution Source Monitoring Systems, pp. 6-1 to 6-18 (Jun. 1979).
.
"Internal Dilution System: AR-120", Anarad, Inc., Santa Barbara,
California (no date)..
|
Primary Examiner: Chilcot; Richard
Assistant Examiner: Noori; Max H.
Attorney, Agent or Firm: Buchanan Ingersoll, P.C. Alstadt;
Lynn J.
Parent Case Text
RELATED APPLICATION
This application is a continuation-in-part of U.S. patent
application Ser. No. 08/218,563, filed Mar. 28, 1994, which is a
continuation-in-part of 07/789,935, filed Nov. 12, 1991, now U.S.
Pat. No. 5,297,432, issued Mar. 29, 1994.
Claims
We claim:
1. An apparatus for collecting a sample stream from a system
containing water vapor and at least one other gas comprising:
a) a collection probe adapted to draw the sample stream from the
system;
b) a particulate filter connected to the collection probe so that
the sample stream will flow therethrough;
c) a conduit connected at one end to the particulate filter and
suited for connection at another end to an analyzer, the conduit
having an orifice sized to permit the sample stream to flow through
the conduit at a sampling rate of from 5 cc. to 250 cc. per minute;
and
d) at least one pump connected to the conduit for drawing a vacuum
on the sample stream.
2. The apparatus of claim 1 also comprising a dryer connected to
the conduit prior to the orifice for removing water from the sample
stream before the sample stream is transported to an analyzer.
3. The apparatus of claim 2 wherein the dryer contains a copolymer
of tetrafluoroethylene and
perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid.
4. The apparatus of claim 2 also comprising a dryer purge line
connected to the dryer and having a critical orifice therein, the
critical orifice positioned within the conduit and the critical
orifice positioned within the purge line being sized so that a
known stable ratio is formed between purge flow and sample
flow.
5. The apparatus of claim 4 also comprising means for supplying a
dry air stream having a known moisture content to the dryer into
which dry air stream moisture removed from the sample stream by the
dryer is exhausted thereby forming a dryer exhaust stream and means
for measuring moisture content of the dryer exhaust stream, both
means being connected to the dryer, such that moisture content in
the sample stream may be calculated from the moisture content of
the dryer exhaust stream and the known stable ratio of sample flow
and purge flow rates.
6. The apparatus of claim 5 wherein the means for supplying a dry
air stream having a known moisture content to the dryer is at least
one of a dryer having an ambient air input and a desiccant.
7. The apparatus of claim 1 also comprising an analyzer connected
to the conduit.
8. The apparatus of claim 7 wherein the analyzer is able to measure
at least one property of the sample stream which property is one of
O.sub.2 level, NOx, SO.sub.2, CO.sub.2 and dew point.
9. The apparatus of claim 1 wherein the at least one pump creates a
pressure in the sample stream not greater than 0.40
atmospheres.
10. The apparatus of claim 1 wherein the filter is a replaceable
cartridge type filter.
11. The apparatus of claim 1 wherein at least a portion of the
conduit is tetrafluoroethylene.
12. The apparatus of claim 1 also comprising a pressure transducer
connected to the conduit.
13. The apparatus of claim 1 also comprising a calibration gas
source connected through a calibration gas conduit to the filter in
a manner so that calibration gas may flow through the filter into
the conduit.
14. The apparatus of claim 13 also comprising a pressure transducer
connected to the calibration gas conduit.
15. The apparatus of claim 13 also comprising an accumulator
connected to the calibration gas conduit.
16. A method of extractive sampling from a system containing water
vapor and at least one other gas comprising the steps of:
a) placing a sampling probe in the system;
b) drawing a vacuum on the sampling probe to collect a sample and
to induce the sample to flow from the system at a sampling rate of
from 5 cc. to 250 cc. per minute;
c) transporting the sample to at least one analyzer through an
elongated conduit and
d) further adjusting the vacuum on the sample to maintain a dew
point of the sample within a preselected temperature range.
17. The method of claim 16 wherein the sample drawn from the system
does not exceed 150 cc./min.
18. The method of claim 16 also comprising the step of measuring a
dew point of the sample.
19. The method of claim 16 also comprising the step of removing
moisture from the sample prior to drawing the vacuum.
20. The method of claim 19 wherein moisture is removed from the
sample by a dryer having a purge line having a purge line critical
orifice therein and the sample is drawn through a sample critical
orifice, the purge line critical orifice and the sample critical
orifice being sized so that a stable ratio is formed between purge
flow and sample flow.
21. The method of claim 19 also comprising the step of measuring
the moisture removed from the sample.
22. The method of claim 19 wherein at least 99% of the moisture in
the sample is removed.
23. The method of claim 19 also comprising the step of measuring a
dew point of the sample.
24. The method of claim 16 wherein the sample is transported under
vacuum at an absolute pressure of less than 20 millimeters of
mercury.
25. The method of claim 24 wherein at least one analyzer is used to
withdraw a portion of the sample from the vacuum conduit and to
measure at least one constituent of the sample.
26. The method of claim 25 wherein the at least one analyzer is a
mass spectrometer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to extractive gas sampling systems
such as are used for analysis of process gases or fossil fuel
combustive gases being vented through a stack.
2. Description of the Prior Art
An important category of extractive gas sampling relates to the
compliance monitoring requirements enforced by the United States
Environmental Protection Agency (EPA). Many sources of air
pollution, such as fossil-fueled power plants, incinerators, metal
smelters, and cement kilns, are required to monitor levels of
certain gaseous species that are released into the atmosphere.
These species include sulfur dioxide, nitrogen oxides, carbon
monoxide, carbon dioxide and oxygen. The EPA standards for
compliance monitoring systems are delineated in Volume 40 of the
Code of Federal Regulations.
The gas streams to be monitored typically have certain intrinsic
characteristics which complicate testing. For example, they
generally contain 6% to 20% by volume of evaporated moisture, which
results in a sample dew point well above that of normal ambient
temperatures. Also, the gas streams often contain significant
amounts of condensed moisture in the form of entrained water
droplets and fog. Acid gases, such as sulfur dioxide are also
generally present. Additionally, the gas streams invariably contain
large quantities of particulate debris such as soot, fly-ash from
fossil fuels and process material.
In order to analyze a sample for its gaseous constituents, it is
necessary to remove the particulates and transport the sample to a
remote location suitable for the operation of gas analysis
instrumentation. For accurate measurements and for reliability of
the test equipment, it is necessary to ensure that moisture and
gases will not condense either in the sample probe, the sample
lines, or the analyzers. However, the methods used to accomplish
these goals must not themselves alter the samples in a way that
negatively impacts testing accuracy.
In the past, two basic types of sampling systems have been
developed for analysis of gaseous mixtures. The first type, the
traditional extractive system, is shown in FIG. 1. Many vendors
have supplied similar systems over the years. This system, however,
has proved to have many undesirable drawbacks as described below.
The second type, illustrated in FIG. 2, is a venturi dilution probe
system. This type of system was developed in the 1980's primarily
in response to perceived inadequacies with the traditional system.
As discussed more fully below, however, the venturi probe system is
also not without disadvantages.
SUMMARY OF THE INVENTION
A gas sampling system practicing the present invention utilizes a
dryer adjacent a sampling probe. The dryer preferably is the type
of dryer which contains a copolymer of tetrafluoroethylene and
perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid. This type of
dryer enables the operator to determine how much moisture was
removed from the sample. A vacuum pump maintains a substantial
vacuum on a conduit which extends from the dryer to an analyzer.
The dew point of the sample within the conduit line is affected by
both the amount of moisture removed by the dryer and the degree of
vacuum transporting the gas mixture. As such, the dew point can be
varied practically indefinitely through an optimum combination of
moisture removal by the dryer and vacuum pump strength.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a prior art system utilizing a heated sample
line for transporting the sample.
FIG. 2 is a diagram of a prior art system utilizing a dilution
probe.
FIG. 3 is a diagram of a present preferred embodiment of the vacuum
dilution extractive gas sampling system of our U.S. Pat. No.
5,297,432.
FIG. 4 is a diagram of a present preferred embodiment of the vacuum
extraction sampling system of the present invention.
DETAILED DESCRIPTION
Prior Art Systems
FIG. 1 illustrates the traditional extractive system. Pump 10 draws
gas through heated probe 12 from a gas stream moving within stack
14 as shown by arrow A. The sample is then transported to a remote
location through a heat-traced sample line 16. Typically, probe 12
and sample line 16 are heated to about 250.degree. F. to prevent
condensation of the moisture or acid in the sample. Next, the
sample is drawn through a "chiller" 18 which lowers the sample
temperature to approximately 35.degree. F. The water vapor thus
condenses and is drained away at drain 20. The sample, now dry, is
then reheated and transported through analyzer 22 which measures
the constituents of interest. The gas sample is maintained at or
near atmospheric pressure during all of this process.
A number of minor variations have been made on this basic design.
Sometimes the pump is located before the analyzer. Sometimes the
gas sample is diluted via the addition of nitrogen or air prior to
analysis for the purpose of bringing the sample concentration
within the range of the analyzers or for the purpose of reducing
certain interferences within the analyzers. The analyzer and sample
pump are sometimes heated so that the chiller may be omitted.
This traditional design presents a number of drawbacks and
limitations. First, in order to move the sample to a remote
location within an acceptable time period (EPA requires a 15 minute
system response time, and process applications demand even a
quicker response) the sample must be large--typically in the order
of two to five liters per minute. Because the amount of particulate
associated with such large samples would quickly clog any fine
filter, only a coarse filter 24, such as the type constructed of
sintered metal or ceramic, can be used. Even a coarse filter,
however, will tend to clog every few hours in this system. To clean
the filter, a blow-back design is required. For this purpose,
compressed air source 26 feeds surge tank 28 which is located near
ball valve 30. When valve 30 opens pressurized air in tank 28 is
released, thereby purging filter 24 of impurities. Since valve 30
is continually exposed to the stack sample, however, it can develop
leaks which distort the sample.
This design also requires use of large amounts of calibration
("cal") gas. Cal gas is a gas sample containing a known
concentration of the species to be measured. This is used to run a
calibration check on the accuracy of the measuring equipment. The
EPA requires that such a calibration check be run daily using
"Protocol-1" gases that may typically cost $400 for a small bottle.
A similar technique using "zero gas" is sometimes employed to null
the species detectors. Referring again to FIG. 1, the cal gas is
fed in the traditional design from cal gas source 32 through line
34 to a location on probe 12 which is behind filter 24. Thus,
deleterious effects of filter 24 such as scrubbing of sulfur
dioxide by alkali particles thereon are not checked by the cal gas.
In addition, a large volumetric flow of cal gas greater than the
volumetric flow through tube 16 is required.
The design has a number of "weak links", which make it inherently
unreliable. For example, if chiller 18 fails, analyzer 22 and pump
10 will likely be destroyed. Additionally, failure of heat tracing
sample line 16 will result in condensation and contamination that
can necessitate replacement of the line and all downstream
plumbing. Heat traced line is significantly more expensive than
unheated line. Also, since ball valve 30, analyzer 22 and pump 10
are exposed to high levels of acidic gases and to the fine
particulates which permeate coarse filter 24, the service life of
these components is reduced considerably.
Furthermore, when this design utilizes a chiller, a serious
measurement methodology problem is presented. Specifically, gas
concentrations are measured on a dry basis (i.e. with the moisture
removed). Pending EPA regulations strongly favor making the
concentration measurement on a wet basis (including vapor-phase
moisture).
The second general type of prior art system, the dilution probe, is
depicted as FIG. 2. In this design, the rate of stack sample
extraction is considerably smaller than is the case with the
traditional system of FIG. 1. Here, gas is drawn through a fine
filter 35 into a device known as a "sonic orifice" or
"critical-flow orifice." Sonic orifice 36 is so called because it
meters a constant volumetric flow provided that a substantial
vacuum exists behind the orifice. Stated another way, a pressure
drop of greater than two to one (2:1) thereacross will induce a
generally constant volumetric flow as metered from the upstream
side of the orifice. Orifice 36 can typically be sized to permit
flow as low as 20 cc per minute and as much as 200 cc per minute.
Vacuum on the back side of orifice 36 is maintained by a venturi 38
which is driven by compressed air source 40. Venturi 38 also serves
to provide clean, dry dilution air which lowers the sample point.
The entire venturi/orifice assembly is constructed within nonheated
probe 42 such that the dilution is accomplished at essentially
stack temperature. The diluted sample is then sent to analyzer 43
at approximately atmospheric pressure.
This technique overcomes some of the deficiencies of the
traditional extraction system. For example, cal gas 44 and zero gas
45 may be introduced upstream of filter 35 which will allow
checking of deleterious filter effects. However, significant
drawbacks remain. For example, because orifice 36 is a true
critical-flow device, while venturi 38 is not, the dilution ratio
is a function of temperature. If the process temperature varies
considerably, the probe will need to be temperature controlled.
Additionally, if the gas stream being sampled is fully saturated,
condensation will occur on filter 35 and orifice 36 before dilution
can occur. In addition, condensation will occur just downstream of
the orifice 36 due to adiabatic cooling of gas passing through. In
these applications, it therefore is necessary to heat the probe
anyway.
Furthermore, in order to prevent condensation in unheated transport
line 46, it is necessary to lower the dew point to below the
expected ambient temperature. In cold climates, dilution ratios of
up to 350:1 are needed. Ratios of this magnitude pose several
problems. First, the concentration of the gas constituents of
interest may be lowered to a level below the sensitivity of
commercially available analyzers. For example, the best carbon
monoxide analyzers can only measure down to five parts-per-million
(5 ppm) with good accuracy. Many facilities must measure actual
stack concentrations of the order of 50 ppm. Stack gas having about
50 ppm of a constituent diluted by the dilution ratio achieved in
the prior art system of FIG. 2 reduces the concentration to well
below 5 ppm. Another problem with high dilution ratios is that the
overall system will become sensitive to minute impurities in the
dilution air. As an illustration, 0.1 ppm of CO in the dilution air
of the above example will be measured by the system as
(350).times.(0.1 ppm), the product of which is thirty-five parts
per million (35 ppm). The analyzer will be unable to differentiate
between this error and a comparable stack level of CO.
Moreover, the only commercially available version of this device
uses a venturi that is operational only with flows of between four
and seven liters per minute. This also poses several problems. For
example, this large a flow of the dilution gas effectively
militates against the use of bottled gas which would be
prohibitively expensive and require frequent maintenance. Thus,
compressed air source 40 is a compressor which utilizes the air in
9 or near the stack. That air contains particulates, CO.sub.2,
SO.sub.x, NO.sub.x and water vapor. Consequently, one must use an
array of dryers 401, scrubbers 402, absorbants 403 and filters 404
to remove contaminants from the dilution gas. Since most analyzers
only require a flow in the order of 0.5 liters/minute, most of the
4-7 liters of diluted sample are wasted. Another problem is that,
for a given sized orifice, there are limits to the dilution ratios
that can be achieved.
Additionally, venturi 38 is generally embedded in a very expensive
probe assembly. Thus, contamination, such as could occur if the
orifice assembly, which is typically made of glass, would break,
necessitates replacement of a very expensive piece.
Gas sampling systems have also been used to evaluate exhaust
emissions from internal combustion engines. Examples of such
systems are disclosed in U.S. Pat. Nos. 3,817,100 to Anderson et
al.; 3,965,749 to Hadden et al.; 4,823,591 to Lewis, and 5,184,501
to Lewis et al. None of these devices are suitable for delivering a
sample over long distances. They also use valve and orifice
combinations to dilute the sample.
Our VDSS System
In our U.S. Pat. No. 5,297,432, we disclose a vacuum dilution
extraction system or VDSS system. The system utilizes a pair of
sonic orifices. One of the orifices provides a constant flow of
sample gas and the other provides a constant flow of dilution gas.
The resulting mixture is transported under substantial vacuum and
repressurized to typically about one atmosphere prior to
analysis.
FIG. 3 illustrates a present preferred embodiment of the VDSS
system. Gas from a system such as a gas stream moving within stack
47 is drawn by sample pump 48 into collection probe 50. The gas is
preferably first filtered. Next, the sample passes through
capillary tube 56 which is within probe 50. After leaving tube 56,
the sample gas enters sample conduit 57 which has sample orifice 58
therein. Dilution gas is preferably simultaneously drawn at a
controlled pressure by pump 48 from dilution gas source 60 into
dilution gas conduit 62 and through dilution orifice 64 therein.
Typical suitable dilution gases may be compressed air, carbon
dioxide and nitrogen, depending on the sample gas and the analyzers
which are desirable to be used. In order for the sample and
dilution gas to be drawn simultaneously, orifices 58 and 64 are
arranged in parallel. Specifically, conduits 57 and 62 intersect
downstream of the orifices, forming mixing conduit 67 where mixing
of the sample and dilution gas occurs. Since pump 48 maintains a
substantial vacuum in conduit 67, the flow rate through orifices 57
and 62 is essentially constant. Thus, a constant dilution ratio is
achieved.
Conduits 57, 62 and 67 may be constructed of any suitable inert
material. Some possible materials for this purpose are glass or a
corrosion resistant metal alloy such as HASTELLOY corrosion
resistant alloys or corrosion resistant polymeric materials such as
TEFLON material. Particularly, HASTELLOY C-22 alloy may be
suitable. Orifice 58 may be as small as 0.0009 inches which
corresponds to a flow rate of 4.2 cc per minute. This is much less
than the minimum 20 cc per minute used in the venturi dilution
system shown in FIG. 2. For a dilution ratio of 25:1, dilution
orifice 64 must be five times larger in diameter than orifice 58,
or 0.0045 inches in this example. This gives a flow rate of 105 cc
per minute of dilution gas if the dilution gas is delivered at
beneficial pressure. If the dilution gas is provided at a higher
pressure, e.g., 14.7 psi, the flow will be 211 cc per minute and
the dilution ratio will be 50:1. As this is a much smaller rate
than the prior art, it is possible to use bottled dilution gas from
gas cylinders instead of plant instrument air or compressor air.
This completely eliminates problems with contamination in the
dilution gas. Furthermore, by suitable selection of the orifices,
it is possible to achieve any desired volumetric dilution ratio
over a range of 1:1 to 250:1. For any specific set of orifices, it
is possible to adjust the volumetric dilution ratio by a factor of
10, that is, over a 1:1 to 10:1 range, by simply adjusting the
dilution gas pressure. Thus, the system enjoys a level of
flexibility previously unattainable.
It is desirable to maintain orifices 58 and 64 in a temperature
stabilized dilution chamber such as heated chamber 66. Chamber 66
is mounted engaging mounting nipple 68 which protrudes from wall 70
of stack 47. If the temperature in the dilution chamber is
maintained at a fairly constant figure, the dilution ratio will be
impervious to stack temperature variation. A temperature of
250.degree. F. has been found suitable for this purpose since it is
well above the dew point of most stack gases.
Typically, the dilution ratio should be chosen such that the dew
point is lowered to below 30.degree. F. Dew points below this
temperature are generally not harmful to the analyzing equipment
since such equipment operates at a higher temperature. Thus,
condensation within the analyzer range will not occur. Generally,
dilution ratios between 10:1 and 50:1 will accomplish this dew
point lowering. However, a dilution ratio of even 50:1 will
generally not alone lower the sample dew point enough for use in a
cold climate since conduit 67 may frequently be exposed to
temperatures below 30.degree. F. For this reason, sample pump 48
transports the mixture under a substantial vacuum (0.15 atmospheric
pressure) which further lowers the dew point of vapor by the factor
(1/0.15)=6.67. The combination of actual volumetric dilution in
vacuum transport makes it possible to lower the dew point below the
coldest expected ambient temperature without actually reducing the
relative concentration of the species of interest to the
undesirably low levels of the prior art devices. Specifically, it
is generally possible to easily lower the dew point to -25.degree.
F. using this technique.
The sample mixture next enters the vacuum side of sample pump 48
and exits the pressure side of sample pump 48 at essentially
atmospheric pressure. It is not essential, however, that the
analyzer 69 be placed on the pressure side of pump 48 and in other
applications it may be desirable to place the analyzing equipment
on the vacuum side of the pump. Thus, the species of interest are
presented to analyzer 69 at a dilution ratio of only approximately
50:1 in the present example. To get the same dew point lowering
with the prior art devices, the sample would have been presented to
the analyzer with a dilution ratio of approximately
50:1.times.6.67=333. This would be an unacceptably high dilution
ratio in many cases for the reasons discussed above. Another
advantage of transporting the sample under substantial vacuum is
that rapid movement of the sample may be accomplished with a
smaller sample rate. That is to say at reduced pressure, a sample
of approximately 211 cc per minute will move through the sample
line as fast as a much larger sample (211.times.6.67) cc/min would
have moved at barometric pressure.
The preferred transport pressure for the system shown in FIG. 3 is
0.15 atmosphere or lower. Commercially available sample pumps could
operate at as low as 0.075 atm. The use of more expensive sample
pumps capable of achieving these lower pressures, however, is only
necessary in the event that: (1) the actual dilution ratio must be
kept low out of a need to operate on the range of a specific
analyzer; and (2) a low ambient temperature is expected. In a warm
climate it will be possible to achieve a sufficiently low dew point
in conduit 67 with both a low dilution ratio and a relatively
inexpensive pump. For purposes of achieving critical flow the pump
only needs to achieve a vacuum of approximately 0.4 atm
pressure.
There are a number of applications in which dilution systems are
not a practical alternative. These include situations where it is
necessary to measure oxygen (which would require use of oxygen-free
dilution gas) or low levels of carbon monoxide (which taxes the
lower-sensitivity channels of analyzers when the sample is
excessively diluted).
Conventional "full-extraction" systems have a deserved reputation
as high-maintenance devices. Analysis of the design of these
systems reveals that nearly all of the maintenance issues originate
from one key problem: in order to minimize lag time in transport,
conventional systems draw too large a sample. Several problems
result. The sample rate, which is typically 3-5 liters per minute,
is too large. Consequently, the front-end filter cannot be very
efficient, or it will clog. There is no way to know whether the
probe blowback cleaning cycle has functioned correctly. Fine
particulates penetrate the system and eventually degrade sample
lines and analyzers. An excessively large sample cannot be dried at
the stack probe location. This necessitates a heat-traced sample
line, with disastrous downstream effects if the heat-tracing fails.
If a refrigerated chiller is used to dry the sample, additional
risk is introduced were the chiller to fail. If a dryer is used to
dry the sample, it can be clogged with particulates. There is no
way to continuously monitor the efficiency of the dryer.
The present vacuum extraction sampling system eliminates all these
problems by providing a system that operates at a greatly reduced
sampling rate, that can dry the sample at the stack, that provides
a stack moisture measure in addition to the other required gases,
and that provides complete diagnostic and interlock protection for
the entire sampling train, drying system and blowback cycle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In our vacuum extraction sampling system shown in FIG. 4 we provide
a probe 12 which extends from a flange 13 attached to the wall 14
of a duct or stack. Preferably the probe is a heated probe which
has been heated to a temperature which is approximately 20.degree.
F. above the stack temperature to prevent condensation. The probe
also should be inclined downward about 10.degree. from a line
perpendicular to the stack wall 14 as shown to prevent rain or
entrained water from being sampled. It should be noted that there
is no filter at the end of the probe and hence no opportunity for
sample "scrubbing" by a filter that is not tested by the
calibration gas supplied by bottles 32. The calibration air supply
32 preferably are bottles of compressed air having a known level of
02 which level has been certified by the supplier. Solenoid values
31 and a filter 33 are provided within the calibration gas supply
line 34. In the present preferred system we also prefer to provide
a pressure transducer 25 and a 2 liter accumulator 27.
Upon exiting the stack, the sample is drawn through sample conduit
1 having a deep-bedded glass wool filter 2, which is backed up by a
porous ceramic filter 3. It is well known that a fibrous filter,
which traps particles through its volume is far more efficient
(>99.995%) than a "surface" filter such as is most commonly used
at the end of full extraction probes. Any filter located at the
probe tip must of necessity be a "surface" type; otherwise, it
could not be cleaned during blowback. Because the present system
utilizes a low sampling rate at about typically 150 cc./min.,
clogging of the filter is not a problem. At this low sampling rate,
a stack grain loading of 100 mg/m.sup.3 will draw only about 22
milligrams of dust into the probe per day. The filter is designed
for 4 to 12 months time between filter changes for gas-fired
applications, and 1 to 3 months for dirtier applications. The
filter cartridge can be changed in a few minutes, without the need
for stopping the sample pump.
The calibration gas for the system passes through the main sample
filter 2, thereby providing a more comprehensive system check that
is possible via traditional probes that have a filter at the probe
tip. All factors that could affect the stack reading, including
scrubbing in the filter, leaks in the transport system, and drift
in the analyzers, are checked during the calibration cycle.
The normal calibration cycle starts with a blowback cycle whereby a
2-liter accumulator of compressed air is blown through the probe in
a few seconds, blowing out any small accumulation of dust that may
be settled in the probe since the last calibration cycle.
Note that there is a pressure transducer 25 in the calibration span
line 34. This device serves two purposes. First, it is used to
verify that following the blowback cycle, the pressure has dropped
to the normal stack level in the expected time. This verifies that
the probe is not clogged. Second, it is used in the normal sampling
mode to monitor the stack pressure.
The sample which is drawn by the probe 12 passes from the filters 2
and 3 into a dryer 4. We prefer to use a dryer which contains a
copolymer of tetrafluroethylene and
perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid. This type of
dryer will remove known quantities of moisture from a sample
stream. Such a dryer is commercially available and sold under the
trademark "PermaPure" by Perma Pure, Inc.
A sample stream is drawn into the system by vacuum pump 10
connected to the dryer 4 through conduit 5 containing sonic orifice
11. Sonic orifice 11 is sized to provide a small sample flow rate
of between 5 cc. to 250 cc. per minute. For stack gas sampling we
prefer a sampling rate of 150 cc./min. Conduit 5 is connected to
the O.sub.2 and NOx analyzers 51 and 52. In nearly all applications
these analyzers operate continuously analyzing the small sample
drawn by the sampling system. We prefer that the pump draw
sufficient vacuum to create a pressure of approximately 0.2
atmospheres. The dryer 4 preferably will lower the dew point of the
sample to about 0.degree. F. The combined effect of the dryer and
the vacuum preferably will lower the dew point of the sample in the
conduit 5 to less than -20.degree. F.
There is membrane 7 within the dryer 4 through which moisture is
removed from the sample. The dryer requires a flow of dry air to
remove the water from the dryer. In the embodiment of FIG. 4 we
provide an ambient air intake 53. Ambient air enters intake 53 and
passes through a particulate filter 54 and sonic orifice 59 into
dryer 55. Dryer 55 can be the same type of dryer as the dryer 4
which is used in the sample intake line 5. Dry air from dryer 55 is
directed through an indicating back up desiccator 61 to the
moisture removal side of dryer 4. We prefer to set the dryer 55 to
dry the filtered ambient air to 10.degree. F. so that after passing
through the indicating desiccator 61, the dew point of the air
stream entering dryer 4 is at a preselected level, preferably,
-100.degree. F. Use of the dryer 55 prior to the indicating
desiccator 61 reduces the replacement interval for the indicating
desiccator 61 from every few days without the dryer to only a few
times per year with the dryer. We provide a dew point monitor 9
connected by valve 21 to conduit 6 to measure the dew point of the
air stream from dryer 4 which contains the moisture that the dryer
4 has removed from the sample stream. From this information it is a
simple matter to calculate the amount of moisture that the dryer 4
has removed from the sample stream and thereby determine the
moisture content of the stack gasses which have been sampled.
Valves are also provided to connect the dew point monitor to the
dried ambient air input line 15 and exhaust conduit 17. In the
preferred embodiment, the dew point analyzer can, via selection of
the appropriate solenoids, monitor any one of three streams: (1)
the purge stream from the dryer in conduit 5; (2) the dew point of
the dried sample in conduit 17; and (3) the dew point of the air
being supplied to dryer 4 through conduit 15.
The present system provides a relatively small sample of from 5 cc.
to 250 cc. per minute. Typically, the system will draw about 150
cc. of stack gases per minute. The relatively small sample used by
the system, along with the very high efficiency of the filter,
makes it possible to dry the sample right on the stack, using a
commercially available "PermaPure" dryer. Such dryers have been
used on extractive systems in the past, but have had the problem of
clogging due to the largevolume, semi-filtered nature of the
sample. This problem is eliminated in the clean, low-flow sample
provided by the present system. Typically, the sample is dried to
0.degree. F. right at the stack, eliminating the need for heated
sample lines and for refrigerated chillers on the ground.
One of the reasons that traditional extractive system required
large sample rates was to reduce transmit time in the sample line.
Our system accomplishes rapid sample transport by moving the system
under vacuum, typically about 0.2 atmosphere. This transports the
150 cc./min. sample five times faster than it would move at
barometric pressure. Using a 0.25" O. D. Teflon tube with 0.0345"
wall thickness, the sample is transported at rates in excess of 125
ft./min.
Vacuum transport of the dried sample serves a second purpose. It
further reduces the sample dew point. A sample that has been dried
to 0.degree. F. at atmospheric pressure will have a dew point of
-28.degree. F. at a transport pressure of 0.2 atmosphere.
Commercially available NOx and O.sub.2 analyzers have a
sufficiently fast response time to meet EPA requirements with
sample flows of less than 100 cc/min. A greater than necessary
sample flow, particularly if the larger sample is at the expense of
thorough filtering, will only cause accelerated contamination of
the sample cells of the analyzer. The present sampling system is
designed to best match the minimal flow requirements of the
analyzers.
The "PermaPure" dryer 4 on the stack can remove more than 99% of
the moisture from the stack sample. The purge flow through conduit
57 is metered through a temperature-controlled orifice 59.
Preferably orifice 59 and orifice 11 in sample line 5 are sized so
that the sample stream flow and the dryer purge stream flow form a
stable ratio. Because the purge flow through the dryer is metered
and therefore known, the absolute amount of removed moisture is
known. Since the stack sample rate is also known, it is
straightforward for this system to calculate the stack moisture
from the known amount of water removed from a known flow. It is
important to know the moisture any time that a mass emission (e.g.
lb/hr) is being calculated from a wet-based flow measurement and a
dry-based gas concentration.
The present system maintains an ongoing self-diagnostic check of
the drying system. Dry air for the stack "PermaPure" dryer 4 is
produced from ambient air via a second "PermaPure" dryer 55 located
in the instrument cabinet. The dew point analyzer 9, which normally
monitors the stack moisture, is periodically switched to check the
dryness of this purge air. It is hereby straightforward to provide
a Maintenance Alert whenever the dew point of this air is out of
tolerance. A canister 61 of indicating desiccant downstream of the
dryer 55 provides temporary protection while the cause of dryer
failure is investigated. An optional "bottle backup" can be
installed that automatically switches to a bottle of dry air when
needed without any interruption in stack monitoring.
A third use of the dew point analyzer 9 is for periodic checks of
the dryness of the stack sample itself. If the dryness of the stack
sample that is being delivered to the analyzers is ever out of
specification, the sampling pump is simply shut off, and the probe
is put into a zero purge mode. No other extractive sampling system
provides such a comprehensive degree of protection for the
analyzers and the sampling line from the deleterious effects of
condensation.
Absolute pressure transducers 12 are located on the vacuum sample
line 5 and on the vacuum side of each of the "PermaPure" dryer
purge lines 6 and 8. Any a time that the vacuums are out of
specification, it is straightforward to provide a Service Alert to
the user. Any time that a measured parameter is dangerously out of
specification, the pump is automatically shut off and the system is
put into the zero purge condition so as to protect the analyzers
from damage.
A further pressure transducer 25 is the one already described in
the calibration span line 3. Any time that the probe pressure does
not return to normal within a specified time, a Probe Service alert
call be provided. This is not a situation that is expected to
occur; inasmuch as only a tiny amount of dust will enter the heated
probe during the period between blowback. In short, the present
system is designed for low sample rate, corresponding low
maintenance, probe blowback, a calibration check that tests the
entire system, monitoring of stack moisture, monitoring of the dew
point of all flowing streams; and monitoring of the
pressure/vacuums of the sample and vacuum flows monitoring for
probe pluggage.
In the embodiment shown in FIG. 4 we show a NO.sub.x analyzer and
an O.sub.2 analyzer. However, other types of analyzers such as a
CO.sub.2 analyzer, could be used in place of or in addition to the
analyzers we have illustrated. In an alternative embodiment we can
provide a mass spectrometer 63, shown in chainline in FIG. 4, in
the vacuum line 5 which can detect the presence of a variety of
chemical compounds in the sample. When a mass spectrometer is used
we prefer that the sample be transported under vacuum at an
absolute pressure of less than 20 millimeters of mercury.
The various components of our system should be made from materials
which are compatible with and not adversely affected by the gases
that are being sampled. Such materials include corrosion resistant
metal alloys, glass, and corrosion resistant polymeric
materials.
We expect that a dryer will be used in most applications of our
system. However, in certain applications it may not be necessary
that the dryer remove 99%, or even 90%, of the water vapor from the
sample. If the sample transport conduit is not subject to extreme
cold, and if a sufficiently high vacuum is used, it may not even be
necessary to use a dryer at all. Commercially available vacuum
pumps can draw a vacuum as low as a fraction of a millimeter of
mercury absolute pressure. Thus, in some situations the vacuum pump
can draw a low enough vacuum to reduce the dew point to an
acceptable level without drying the sample. The essence of the
present invention is that, by use of a small sample and vacuum
transport, commercially available dryers may be used when needed,
such that the sample may be transported through unheated conduits
without the need for dilution.
Although we have shown and described certain present preferred
embodiments of our vacuum sampling system, it is to be distinctly
understood that our invention is not limited thereto but may be
variously embodied within the scope of the following claims.
* * * * *